Method for producing separator of fuel cell

ABSTRACT

The present invention provides a method of producing the separator for a fuel cell that is suitable for mass production and that by separating the compression-molding process of pressing a precursor powder comprising carbon powder and thermosetting resin with a press from the resin-setting process of setting the thermosetting resin by applying heat, is capable of increasing the production rate of the expensive press, while at the same time decreasing the production cost. The fuel-cell separator is produced by a compression-molding process in which a mixture of carbon powder and thermosetting resin is used as the precursor powder, and in which a press is used to apply pressure to the precursor powder that has been placed in a die and mold it into a molded plate having a separator shape; and a resin-setting process in which the molded plate formed in the compression-molding process is heated in a heating apparatus such as an oven to set the thermosetting resin.

BACKGROUND OF THE INVENTION

1. Field of the invention

This invention relates to a method for producing the separator of a fuelcell that is used in a solid-polymer-type fuel cell, and moreparticularly to a method of producing the separator of a fuel cell thatuses a mixture comprising a carbon powder and thermosetting resin as theprecursor powder.

2. Related Art of the Invention

A fuel cell is an apparatus that allows hydrogen and oxygen to react byway of an electrolyte to convert chemical energy directly to electricenergy, and there are several types depending on the electrolyte used,however, since a solid-polymer-type fuel cell that uses asolid-polymer-electrolyte film as the electrolyte can operate at lowtemperature of about 80° C., it is attracting attention as a generatingapparatus for electric automobiles.

FIG. 4 is an exploded view showing the construction of a unit cell of afuel cell, and FIGS. 5A and 5B are drawings showing the construction ofthe separator of the fuel cell shown in FIG. 4, where FIG. 5A is a topview and FIG. 5B is a cross-sectional view of section X-Y shown in FIG.5A.

As shown in FIG. 4, a solid-polymer-type fuel cell is constructed byarranging tens to hundreds of MEAs (membrane electrode assemblies)comprising two fuel-cell separators 1, which have a plurality of grooveson both the left and right surfaces, and that by way of a gasket 5 jointogether a solid-polymer-electrolyte film 2, anode (fuel electrode) 3and cathode (oxidizing-agent electrode) 4 as a unit cell; where bysupplying fluid fuel gas (hydrogen gas) to the anode and fluid oxidationgas (oxygen gas) to the cathode, electric current is taken from theexternal circuit.

As shown in FIG. 5A and 5B, the fuel-cell separator 1 is shaped suchthat it has a plurality of gas-supply/discharge grooves 11 on one sideor both sides of a thin plate-shaped body, opening sections 12 throughwhich fuel gas or oxidation gas is supplied to the gas-supply/dischargegrooves 11, and fastening holes 13 for lining up the MEA in parallel;and together with having the function of separating the fuel gas andoxidation gas that flow through the fuel cell such that they do not mixwith each other, it also has the important role of transmitting theelectric energy generated by the MEA to the outside, and to radiate heatthat is generated by the MEA to the outside.

Therefore, characteristics that are desired in a fuel-cell separator 1are bolt tightening during assembly and sufficient strength againstvibration of an automobile or the like, decreased electrical resistancein order to decrease power generation loss, and gas impermeability inorder to completely separate the supply fuel gas and oxidation gas onboth sides and supply the gases to the electrodes.

For this kind of fuel-cell separator 1, a carbon compound material thatuses a thermosetting resin such as phenol resin, which is veryadvantageous from both a production and cost aspect, has been proposed(for example, Japanese Patent No. S59-26907), and as the productionmethod, a method of mixing carbon powder and thermosetting resin andplacing it in a die, then compressing it while at the same time heatingit to harden the resin has been used.

However, in this prior art, when manufacturing a fuel-cell separator 1using a precursor powder comprising carbon powder and thermosettingresin, the precursor powder is placed in a die, and using a press,pressure and temperature are applied, and even though phenol resin is afast setting thermosetting resin, it requires several minutes to set at160° C., and depending on the size of the precursor powder, severalminutes are also required for the heat to be transferred to theprecursor powder and for the temperature to rise; therefore, a problemexists in that the production speed of the expensive press equipment isslowed.

Taking the aforementioned problems into consideration, the object of thepresent invention is to provide a method for producing the separator fora fuel cell that is suitable for mass production and that makes itpossible to increase the production rate of an expensive press, andreduce the production cost by separating the compression moldingprocess, in which a press is used to compress a precursor powder thatcomprises carbon powder and thermosetting resin, from the resin-settingprocess of setting thermosetting resin by adding heat.

SUMMARY OF THE INVENTION

In order to solve the aforementioned problems, the present invention isconstructed as described below.

The invention according to a first claim of the invention is a method ofproducing the separator of a fuel cell that uses a carbon powder andthermosetting resin mixed at a specified ratio as the precursor powder,and comprising: a compression-molding process of placing the precursorpowder in a die and performing cold-compression molding; and aresin-setting process of applying heat to the thermosetting resin at theresin-setting temperature or greater with no pressure being applied tothe precursor powder that was molded in the compression-molding process.

The invention according to a second claim is the method of producing theseparator of a fuel cell of claim 1 wherein in the compression-moldingmethod a pressure of 100 MPa or more is applied to the precursor powder.

The invention according to a third claim is the method of producing theseparator of a fuel cell of claim 1 or claim 2 wherein phenol resin isused as the thermosetting resin and the carbon powder is coated withthat phenol resin.

The invention according to a fourth claim is a fuel-cell separator thatis produced using the method of producing the separator of a fuel cellof any one of the claims 1 to 3.

The invention according to a fifth claim is a fuel cell in which aplurality of unit cells, which comprise an anode and cathode that arejoined by a pair of separators by way of a solid-polymer-electrolytemembrane, are arranged in parallel, and that uses the fuel-cellseparator of claim 4 for all or part of the separator.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is schematic drawing showing the construction of a press that isused in the compression molding process of the method for producing theseparator of a fuel cell of this invention.

FIG. 2 is a schematic drawing showing the construction of the heatingapparatus that is used in the resin-setting process of the method forproducing the separator of a fuel cell of this invention.

FIG. 3 is a table showing the precursor powder, process and adequacyjudgment results for embodiments and comparative examples of the methodfor producing the separator of a fuel cell of this invention.

FIG. 4 is an exploded view showing the construction of a unit cell of afuel cell.

FIGS. 5A and 5B are drawings showing the construction of the separatorof the fuel cell shown in FIG. 4, where FIG. 5A is a top view and FIG.5B is a cross-sectional view of section X-Y shown in FIG. 5A.

DETAILED DESCRIPTION OF THE INVENTION

The preferred embodiment of the invention will be explained in detailbelow based on the drawings.

FIG. 1 is schematic drawing showing the construction of a press that isused in the compression molding process of the method for producing theseparator of a fuel cell of this invention, and FIG. 2 is a schematicdrawing showing the construction of the heating apparatus that is usedin the resin-setting process of the method for producing the separatorof a fuel cell of this invention.

This embodiment of the invention is a method for producing a fuel-cellseparator 1 comprising: a compression-molding process that uses amixture of carbon powder and thermosetting resin as the precursorpowder, and that by using a press to apply pressure to the precursorpowder that has been placed in a die, it compresses and molds it into amolded plate having the separator shape; and a resin-setting processthat heats the molded plate formed in the compression-molding process ina heating apparatus such as an oven to set the thermosetting resin.

First, the carbon powder and thermosetting resin are prepared at aspecified weight ratio, and sufficiently mixed to create the precursorpowder. It is preferred that the precursor powder be mixed at a weightratio of 90% to 60% carbon and 10% to 40% thermosetting resin. Also, inaddition to the carbon powder and thermosetting resin, it is alsopossible to add as necessary a fiber base material, filler, moldlubricant, anti-hydration decomposer, or the like to the precursorpowder.

It is possible to use graphite powder (carbon powder) or the like as thecarbon powder, and when graphite powder is used, it is preferred thatgraphite power having an average grain size of 10 μm to 100 μm, and anaspect ratio of 2 or less be used.

When a phenol resin such as a resol-type phenol resin, or novolak-typephenol resin is used as the thermosetting resin, moldability is good,and furthermore, when carbon power having a phenol resin coating isused, the strength is increased, which is preferable.

In the compression-molding process, a press such as shown in FIG. 1 isused, and the precursor powder is placed evenly in the die (female die),and by pressing the die (female die) against another die (male die) witha hydraulic cylinder, the female die and male die apply a pressure of100 MPa or more to the precursor powder to form a separator-shapedmolded plate. At this time, by having specified groove shapes formed onthe pressure surfaces of the female die and male die,gas-supply/discharge grooves 11 are reflected on and formed on themolded plate.

Also, the compression-molding process is cold-compression molding thatis performed at room temperature and does not require heating, and sinceit is not necessary to heat the precursor powder, it is possible toreduce the molding time, making it possible to compress and mold oneseparator-shaped molded plate in a time of 5 to 10 seconds. Also, sincethe resin is not set, the die does not adhere to the product, so thereis good mold release.

When performing cold-compression molding with a press, it is possible toobtain a high-density separator-shaped molded plate by applying apressure of 100 MPa or more to the precursor powder, and thus it ispossible to produce a fuel-cell separator 1 that has both good massproductivity and high performance.

In the resin-setting process, the molded plate that is taken out of thedie is placed inside a heating apparatus as shown in FIG. 1, and using amethod of heating such as an electric heater, heat is applied in a stateof no applied pressure at a temperature of 170° C. to 200° C., which isgreater than the setting temperature of the thermosetting resin, to setthe thermosetting resin and produce the fuel-cell separator 1. In thisresin-setting process, it is possible to place a lot of molded plates ina batch-type oven at the same time and heat them, or in the case of acontinuous oven, it is also possible to place the molded plates on abelt conveyor and heat them. In either case, heating takes time,however, in these methods it is possible to process a large quantity,and thus the total production time is reduced.

As was explained above, with this embodiment, by separating thecompression-molding process of molding a precursor powder comprisingcarbon powder and thermosetting resin using a press from theresin-setting process of the thermosetting resin by applying heat, it ispossible to reduce the amount of time required since it is not necessaryto heat the precursor powder in the compression-molding process, andsince it is possible to mold one separator-shaped molded plate in a timeof 5 to 10 seconds, this embodiment is effective in making it possibleto increase the production rate of expensive press equipment.

Furthermore, with this embodiment, by separating the compression-moldingprocess of molding a precursor powder comprising carbon powder andthermosetting resin using a press from the resin-setting process of thethermosetting resin by applying heat, it is possible to slowly heat andset a plurality of molded plates that were molded in thecompression-molding process at the same time in a continuous oven or thelike, so this embodiment is effective in making it possible to greatlyreduce the production time as well as cut the production cost, thusmaking it possible to overwhelmingly improve mass productivity.

The preferred embodiments of this invention and comparative examples aredescribed below and used to explain the present invention in detail;however, the present invention is not limited to the embodimentsdescribed below.

FIG. 3 is a table showing the precursor powder, process and adequacyjudgment results for embodiments and comparative examples of the methodfor producing the separator for a fuel cell of this invention.

For each of the embodiments and comparative examples, the density,electrical resistance, airtightness and bending strength are measuredand compared, and the methods used for measuring the density, electricalresistance, airtightness and bending strength are as described below.

Density: The bulk density was calculated from the value obtained bydividing the weight by the volume.

Electrical Resistance: The electrical resistance is measured by the4-terminal method using a molded sample that is processed to a squareshape having a length of 200 mm and 1 mm cross section.

Airtightness: The airtightness was measured according to Method AA ofJIS K7126 (differential pressure method) under the following conditions:

-   Sample air conditioning: 23° C., 50% RH * 48 Hr or more,-   Measurement temperature: 23° C.,-   Used gas: Hydrogen gas.

Bending strength: The bending strength was measured according to ASTMD-790 of a molded sample that was processed to a length of 60 mm, widthof 20 mm and thickness of 3 mm using 3-point bending having a 50 mmspan.

Embodiment 1

Graphite powder having an average grain size of 20 μm and grain aspectratio of 1.5 was used as the carbon powder, and powdered phenol resinwas used as the thermosetting resin, and they were sufficiently mixed ata weight ratio of 85% carbon powder and 15% resin to form the precursorpowder. Then 600 g of that mixed powder was placed evenly in a femaledie having dimensions 300×200×20 mm, and a molded sample was formed byapplying a pressure of 200 MPa by a male die to solidify the powder. Thepressurization rate was 5 mm/sec, and the temperature of the dies andprecursor powder while pressure was being applied was kept at roomtemperature, which was 25° C. The pressed and solidified molded samplewas removed from the die, and the density was measured and found to be1.90 g/cc.

Next, the molded sample that was removed from the die was placed in aheating apparatus and heated up to 200° C. to set the resin. Thetemperature rise at that time was from room temperature to 200° C. over3 hours. After the molded sample was removed from the heating apparatusand observed, it was found that the density had decreased a little to1.87 g/cc, however no warping, deformation, bulging or the like wasobserved. Furthermore, the physical properties were measured and foundto be: electrical resistance=8 mΩcm, airtightness=1.5×10−6 cm3/cm2·s,bending strength=60 MPa, and the performance was also found to besuitable for a fuel-cell separator 1.

Embodiment 2

The same carbon powder and thermosetting resin as used in embodiment 1were used and mixed sufficiently at a weight ratio of 75% carbon powderand 25% resin to obtain the precursor powder. Then 600 g of that mixedpowder was placed evenly in a female die having dimensions 300×200×20mm, and a molded sample was formed by applying a pressure of 100 MPa bya male die to solidify the powder. The pressurization rate was 5 mm/sec,and the temperature of the dies and precursor powder while pressure wasbeing applied was kept at room temperature, which was 25° C. The pressedand solidified molded sample was removed from the die, and the densitywas measured and found to be 1.85 g/cc.

Next, the molded sample that was removed from the die was placed in aheating apparatus and heated up to 200° C. to set the resin. Thetemperature rise at that time was from room temperature to 200° C. over3 hours. After the molded sample was removed from the heating apparatusand observed, it was found that the density was still 1.85 g/cc, and nowarping, deformation, bulging or the like was observed. Furthermore, thephysical properties were measured and found to be: electricalresistance=15 mΩcm, airtightness=4.8×10⁻⁸ cm³/cm2·s, bending strength=65MPa, and the performance was also found to be suitable as a fuel-cellseparator 1.

Embodiment 3

The carbon powder in embodiment 1 was coated with phenol resin (theweight ratio was 85% carbon, 15% resin) and used as the precursorpowder. Then 600 g of that mixed powder was placed evenly in a femaledie having dimensions 300×200×20 mm, and a molded sample was formed byapplying a pressure of 150 MPa by a male die to solidify the powder. Thepressurization rate was 5 mm/sec, and the temperature of the dies andprecursor powder while pressure was being applied was kept at roomtemperature, which was 25° C. The pressed and solidified molded samplewas removed from the die, and the density was measured and found to be1.92 g/cc.

Next, the molded sample that was removed from the die was placed in aheating apparatus and heated up to 200° C. to set the resin. Thetemperature rise at that time was from room temperature to 200° C. over3 hours. After the molded sample was removed from the heating apparatusand observed, it was found that the density decreased somewhat to 1.90g/cc, however, no abnormal warping, deformation, bulging or the like wasobserved. Furthermore, the physical properties were measured and foundto be: electrical resistance=7 mΩcm, airtightness=1.2×10⁻⁶ cm³/cm2·s,bending strength=70 MPa, and the performance was also found to besuitable as a fuel-cell separator 1.

COMPARATIVE EXAMPLE 1

Except that the compression force applied to the dies was 50 MPa, themolded sample was created using exactly the same precursor power andprocess as in embodiment 1. The pressed and solidified molded sample wasremoved from the die, and the density was measured and found to be 1.70g/cc.

Next, the molded sample that was removed from the die was placed in aheating apparatus and heated up to 200° C. to set the resin. Thetemperature rise at that time was from room temperature to 200° C. over3 hours. After the molded sample was removed from the heating apparatusand observed, it was found that the density was still 1.70 g/cc and noabnormal warping, deformation, bulging or the like was observed.Furthermore, the physical properties were measured and found to be:electrical resistance=20 mΩcm, airtightness=3.0×10⁻⁴ cm³/cm2·s, bendingstrength=50 MPa. The airtightness became worse by two digits, and wasnot suitable for use as a fuel-cell separator 1.

COMPARATIVE EXAMPLE 2

Except that the compression force applied to the dies was 75 MPa, themolded sample was created using exactly the same precursor power andprocess as in embodiment 2. The pressed and solidified molded sample wasremoved from the die, and the density was measured and found to be 1.75g/cc.

Next, the molded sample that was removed from the die was placed in aheating apparatus and heated up to 200° C. to set the resin. Thetemperature rise at that time was from room temperature to 200° C. over3 hours. After the molded sample was removed from the heating apparatusand observed, it was found that the density was still 1.75 g/cc and noabnormal warping, deformation, bulging or the like was observed.Furthermore, the physical properties were measured and found to be:electrical resistance=20 mΩcm, airtightness=5.0×10⁻⁵ cm³/cm2·s, bendingstrength=45 MPa. The airtightness became worse and was not suitable foruse as a fuel-cell separator 1.

COMPARATIVE EXAMPLE 3

Except that the compression force applied to the dies was 20 MPa, themolded sample was created using exactly the same precursor power andprocess as in embodiment 3. The pressed and solidified molded sample wasremoved from the die, and the density was measured and found to be 1.72g/cc.

Next, the molded sample that was removed from the die was placed in aheating apparatus and heated up to 200° C. to set the resin. Thetemperature rise at that time was from room temperature to 200° C. over3 hours. After the molded sample was removed from the heating apparatusand observed, it was found that the density decreased somewhat to 1.65g/cc, however, no abnormal warping, deformation, bulging or the like wasobserved. Furthermore, the physical properties were measured and foundto be: electrical resistance=25 mΩcm, airtightness=8.0×10⁻³ cm³/cm2·s,bending strength=38 MPa. The airtightness became worse and was notsuitable for use as a fuel-cell separator 1.

As described above, in embodiments 1 to 3 where the pressure acting onthe precursor powder in the compression-molding process was 100 MPa orgreater, it was possible to obtain a fuel-cell separator 1 that hassufficient physical properties and for which the electrical resistance,airtightness, and bending strength performance are all satisfied,however, in comparative examples 1 to 3 where the pressure acting on theprecursor powder in the compression-molding process was 20 MPa to 75MPa, the airtightness particularly became worse, and it was not possibleto obtain a fuel-cell separator 1 that has sufficient physicalproperties.

The present invention is not limited to the embodiments described above,and it is clear that each of the embodiments can be properly changedwithin the technical scope of the invention. Moreover, the number,location and shape of each of the aforementioned components are notlimited by the embodiments described above, and it is possible to useany appropriate number, location or shape in order to embody the presentinvention. In the drawings, the same reference numbers are used foridentical component elements.

INDUSTRIAL APPLICABILITY

By separating the compression-molding process using a press to compressa precursor powder comprising carbon powder from the resin-settingprocess of the thermosetting resin by applying heat, the method forproducing the separator of a fuel cell of this invention is effective inmaking it possible to reduce the molding time since it is not necessaryto heat the precursor powder during the compression-molding process, andsince one separator-shaped molded plate can be molded in a time of 5 to10 seconds, it is possible to increase the production rate usingexpensive press equipment.

Furthermore, by separating the compression-molding process using a pressto compress a precursor powder comprising carbon powder from theresin-setting process of the thermosetting resin by applying heat, themethod for producing the separator of a fuel cell of this invention iseffective in making it possible to slowly heat and set a plurality ofmolded plates, which were molded in the compression-molding process, ina continuous oven or the like at the same time, so it is possible togreatly reduce the production time and cut the production costs, andthus it is possible to overwhelmingly improve mass productivity.

1. A method of producing the separator of a fuel cell that uses a carbonpowder and thermosetting resin mixed at a specified ratio as theprecursor powder, and comprising: a compression-molding process ofperforming cold-compression molding of said precursor powder that hasbeen placed into a die into a separator-shaped molded plate; and aresin-setting process of heating said molded plate that was molded insaid compression-molding process to the resin-setting temperature orgreater than said thermosetting resin with no applied pressure.
 2. Themethod of producing the separator of a fuel cell of claim 1 wherein insaid compression-molding process, a pressure of 100 MPa or greater isapplied to said molded plate.
 3. The method of producing the separatorof a fuel cell of claim 1 wherein phenol resin is used as saidthermosetting resin and said carbon powder is coated with that phenolresin.
 4. A fuel-cell separator produced using a method that uses acarbon powder and thermosetting resin mixed at a specified ratio as theprecursor powder, and comprising: a compression-molding process ofperforming cold-compression molding of said precursor powder that hasbeen placed into a die into a separator-shaped molded plate; and aresin-setting process of heating said molded plate that was molded insaid compression-molding process to the resin-setting temperature orgreater than said thermosetting resin with no applied pressure.
 5. Afuel cell in which a plurality of unit cells, which comprise an anodeand cathode that are joined by a pair of separators by way of asolid-polymer-electrolyte membrane, are arranged in parallel, and thatuses said fuel-cell separator for all or part of said separator, whereinsaid separator is produced using a method that uses a carbon powder andthermosetting resin mixed at a specified ratio as the precursor powder,and comprising: a compression-molding process of performingcold-compression molding of said precursor powder that has been placedinto a die into a separator-shaped molded plate; and a resin-settingprocess of heating said molded plate that was molded in saidcompression-molding process to the resin-setting temperature or greaterthan said thermosetting resin with no applied pressure.